1. Field of the Invention
This invention relates to a molded thermoplastic closure for a container for the packaging of a product under at least a partial vacuum, which closure has a portion whose position is altered by a loss of vacuum within the container to give a visual indication of such loss of vacuum.
2. Description of the Prior Art
Many food products which are packaged in glass jars are packaged under a partial vacuum to prevent spoilage or to preserve flavor, and it is important that the closure for such a container be able to seal the container properly to maintain the vacuum in the container until the first opening thereof. It has also been recognized that it is desirable for a closure for a container for a vacuum-packed product to incorporate means which will indicate the presence or absence of the desired degree of vacuum, and the prior art is familiar with metal closures which incorporate such a feature. For example, U.S. Pat. No. 4,093,094 (Smalley, et al.), which is assigned to the assignee of this application, describes a home canning closure in which the metal lid of a multi-piece closure incorporates a vacuum indicating button. The button in such a closure is deflected downwardly by the presence of a suitable degree of partial vacuum in the associated container, and because of the inherent elasticity of the closure, the deflected button will pop up to its normal position upon the release of the vacuum and the resulting repressurization of the container. U.S. Pat. No. 3,062,396 (G. J. Foss, et al), 3,836,033 (A. Podesta), and 3,160,302 (G. F. Chaplin), disclose one-piece metallic vacuum indicating closures that operate in a similar manner.
The use of a vacuum button or panel has heretofore been largely limited to metal closures or at least to closures with metal lids, however, and many of the plastic materials which have heretofore been widely utilized in the manufacture of container closures have not incorporated a similar vacuum indicating feature. The reason for this is that such plastic materials tend to be dimensionally stable only when they are under no appreciable load, and the distortion of a vacuum panel or button in the central panel of such a closure by virtue of the vacuum in the associated container would normally cause the central panel to gradually elongate (or creep) under the continued loading resulting from the vacuum. This problem could be aqgravated in the case of a container exposed to abnormally high temperatures, such as, for example, those prevailing in non-air conditioned warehouses during summer months, where filled containers are sometimes stored for prolonged periods of time. In a case such as this a clear pop-up of a stressed vacuum indicating panel in a plastic closure cannot be assured.
Co-pending U.S. patent application Ser. No. 673,063 of James F. Nolan, now U.S. Pat. No. 4,616,761, which is assigned to the assignee of this application, describes a vacuum indicating thermoplastic closure for a container for the packaging of a vacuum packed product. The central panel of the thermoplastic closure of the aforesaid James F. Nolan application has an interior portion which is integrally attached to an inner annular surrounding portion by means of a thin circular region of reduced thickness which acts as a hinge between the interior portion and the inner annular portion, and the central panel also has an outer annular portion and a second thin circular region of reduced thickness which acts as a hinge between the inner annular portion and the outer annular portion. Through the use of two circular hinges, as described, the interior portion is free to move up and down with respect to the rest of the thermoplastic closure under the influence of the vacuum in the associated container, or the lack thereof as the case may be, without developing significant levels of tensile stress in any portion of the closure panel, an important feature in a closure formed from a thermoplastic material, because of the tendency of such materials to gradually elongate (creep) under sustained loading. However, the closure of the aforesaid James F. Nolan application is so free of stress in its vacuum indicating position that there is no positive stress therein to cause the interior portion of the central panel thereof to raise to its vacuum non-indicating position, the movement of such interior portion being left primarily to pressure conditions in the head space of the associated container which can, in certain circumstances, lead to non-failsafe operating characteristics.